This invention relates to elastomer compositions, and is particularly directed to siloxane elastomer compositions capable of forming coatings which are resistant to laser radiation, and to a process for protecting the surfaces of objects, especially aerospace vehicles, from laser radiation and thermal radiation.
Ablative coatings are known for use in various portions of aerospace vehicles to protect the outer skin of an aerospace body such as a missile from the eroding effect of the high temperatures which occur when the vehicle or body passes at high speed through the atmosphere. An ablative coating for this purpose is disclosed in U.S. Pat. No. 3,268,359, prepared from a composition consisting essentially of certain siloxane elastomers. Such siloxanes are disclosed as generally containing fillers which aid in stabilizing the composition. Fillers which are disclosed in the patent include inorganic materials such as fume silica, silica aerogel, diatomaceous earth, quartz, sand, silicates such as aluminum silicate, clay and zirconium silicate and metal oxides such as titanium dioxide and ferric oxide. These are stated to be conventional inorganic fillers normally used in organo-siloxane elastomers.
Recently, high energy infrared lasers have been developed for many applications including the use of high power lasers in laboratories as investigating tools and in research. Devices employing powerful lasers have been developed for communication, range finding and other military purposes. The energy transmitted by these lasers can be very dangerous both to personnel and to sensitive devices such as aerospace vehicles.
Interposing a filter which absorbs or diverts the laser beam has been found to be insufficient in that the filter material is often immediately melted and/or vaporized by the high localized energy application. Even highly efficient reflectors may be destroyed when surface dirt or dust absorbs sufficient energy to cause localized overheating and destruction of the reflector.
U.S. Pat. No. 3,982,206 discloses a system for protection from laser radiation in the form of a protective cover window which comprises a plate which is substantially transparent to infrared radiation, and a vaporizable coating on the plate which comprises a fluorinated ethylene-propylene polymer which rapidly vaporizes without leaving a residue when subjected to intense infrared radiation. Other materials which can be employed include epoxy, silicone and polysulfide resins, the fluorinated ethylenepropylene resins being preferred.
Elastomer materials such as silicone or siloxane elastomers have the advantage of having relatively low thermal conductivities, but have the disadvantage in that they have relatively low ablation energies. Thus, elastomers such as siloxanes are known to have poor laser radiation resistance because of their low ablation energies. Accordingly, an investigation was made for materials which would provide increased laser performance, that is increased resistance from laser radiation, while offering the above noted advantages of the elastomer. In this investigation, it was sought to obtain a composition of a silicone resin and an additive material which when combined is effectively reflective to the incident radiation, so that when the composition or cured coating formed therefrom chars, it does not char black, i.e., becomes absorptive to the radiation, but chars white, i.e., the charred coating retains its reflectivity during the ablation process. At the same time it was necessary that the additive be a high melting material of suitable optical characteristics, and which is not hydrolyzable or have other deleterious physical or chemical properties.
Accordingly one object of the invention is the provision of an elastomer material and an inorganic additive which, when combined, are capable of forming a coating having improved resistance to laser radiation, and to a process for protecting the surfaces of an object from laser radiation and thermal radiation.
Another object is the provision of an elastomer coating of the type noted above comprising a siloxane elastomer and an additive which is effectively reflective to incident laser radiation so that when a coating formed from the composition chars by impingement of laser radiation, the coating chars white, i.e., the coating retains its reflectivity during the ablation process.
A still further object is to provide an elastomer composition of the above type including as essential elastomer component, a siloxane resin and an inorganic additive in an amount capable of forming a cured coating having enhanced resistance to laser radiation and also resistance to thermal radiation sources.
A still further object is the provision of laser resistant compositions of the above type formed essentially of only two components and which is readily formulated and applied as a coating on a substrate, such as the skin of an aerospace body or missile.